(1) Field of the Invention
This invention relates generally to trophic or growth factors and, more particularly, to a new growth factor, artemin, which is a member of the neurturin-persephin-GDNF family of growth factors.
(2) Description of the Related Art
The development and maintenance of tissues in complex organisms requires precise control over the processes of cell proliferation, differentiation, survival and function. A major mechanism whereby these processes are controlled is through the actions of polypeptides known as "growth factors". These structurally diverse molecules act through specific cell surface receptors to produce these actions.
Growth factors termed "neurotrophic factors" promote differentiation, maintain a mature phenotype and provide trophic support, promoting growth and survival of neurons. Neurotrophic factors reside in the nervous system or in innervated tissues. Nerve growth factor (NGF) was the first neurotrophic factor to be identified and characterized (Levi-Montalcini et al., J. Exp. Zool. 116:321, 1951). NGF exists as a non-covalently bound homodimer that promotes the survival and growth of sympathetic, neural crest-derived sensory, and basal forebrain cholinergic neurons. In sympathetic neurons this substance produces neurite outgrowth in vitro and increased axonal and dendritic growth in vivo. (See Levi-Montalcini and Booker, Proc Nat'l Acad Sci 46:384-391, 1960; Johnson et al. Science 210: 916-918, 1980; Crowley et al., Cell 76:1001-12, 1994). NGF has effects on cognition and neuronal plasticity, and can promote the survival of neurons that have suffered damage due to a variety of mechanical, chemical, viral, and immunological insults (Snider and Johnson, Ann Neurol 26:489-506, 1989; Hefti, J Neurobiol 25:1418-35, 1994). NGF also is known to extensively interact with the endocrine system and in immune and inflammatory processes. (Reviewed in Scully and Otten, Cell Biol Int 19:459-469, 1995; Otten and Gadient, Int. J. Devl Neurosci 13:147-151, 1995). For example, NGF promotes the survival of mast cells. (Horigome et al. J Biol Chem 269:2695-2707, 1994).
In recent years it has become apparent that growth factors fall into classes, i.e. families or superfamilies based upon the similarities in their amino acid sequences. These families include, for example, the fibroblast growth factor family, the neurotrophin family and the transforming growth factor-beta (TGF-.beta.) family. As an example of family member sequence similarities, TGF-.beta. family members have 7 canonical framework cysteine residues which identify members of this superfamily.
NGF is the prototype of such a family of growth factors. Brain-derived neurotrophic factor (BDNF), the second member of this family to be discovered, was shown to be related to NGF by virtue of the conservation of all six cysteines that form the three internal disulfides of the NGF monomer (Barde, Prog Growth Factor Res 2:237-248, 1990 and Liebrock et al. Nature 341:149-152, 1989). By utilizing the information provided by BDNF of the highly conserved portions of two factors, additional members (NT-3, NT-4/5) of this neurotrophin family were rapidly found by several groups (Klein, FASEB J 8:738-44, 1994).
Recently, a new family of neurotrophic factors has been identified whose members are not structurally related to NGF and other neurotrophins but are structurally similar to TGF-.beta.. As described in U.S. Pat. No. 5,739,307 and copending application Ser. No. 08/931,858, the known members of this subfamily of the TGF-.beta. superfamily include glial cell line-derived neurotrophic factor (GDNF), neurturin (NTN), and persephin (PSP). The placement of GDNF, neurturin and persephin into the same growth factor family, also referred to as the GDNF ligand family, is based on the similarities of their physical structures and biological activities. Human persephin has about 40% sequence identity and about 43% sequence conservation with human GDNF; and about 49% sequence identity and about 50% sequence conservation with human neurturin. Similarly, human neurturin has about 43% sequence identity and about 53% sequence conservation with human GDNF. In addition, these three proteins have seven cysteine residues whose positions are exactly conserved. GDNF, neurturin and persephin each support the survival of dopaminergic midbrain neurons, and spinal and facial motor neurons, in both in vitro survival and in vivo injury paradigms, identifying these ligands as potential therapeutic agents in the treatment of neurodegenerative diseases (Henderson et al., Science 266, 1062-1064, 1994; Horger et al., J Neurosci. 18, 4929-37 1998; Lin et al., Science 260, 1130-1132, 1993; Milbrandt et al., Neuron 20, 245-53, 1998; Oppenheim et al., Nature 373, 344-346, 1995), reviewed by (Grondin and Gash, J Neurol. 245(11 Suppl 3), 35-42, 1998). However, while GDNF and neurturin both support the survival of many peripheral neurons in culture, including sympathetic, parasympathetic, sensory, and enteric neurons (Buj-Bello et al., Neuron 15, 821-828, 1995; Ebendal et al., J Neurosci Res 40, 276-284, 1995; Heuckeroth et al., Dev Biol 200, 116-29, 1998; Kotzbauer et al., Nature 384, 467-470, 1996; Trupp et al., J of Cell Biology 130, 137-148, 1995), persephin does not share any of these activities on peripheral neurons (Milbrandt et al., supra.
It was recently reported that GDNF and neurturin share receptors and signal transduction pathways (Creedon et al., Proc. Natl. Acad. Sci. US 94:7018-7023, 1997; Durbec et al., Nature 381:789-793, 1996; Trupp et al., Nature 381:785-789, 1996; Baloh et al., Neuron 18:793-802, 1997). These proteins act through a multicomponent receptor complex in which a transmembrane signal transducing component, the Ret protein-tyrosine kinase (Ret or Ret PTK), is activated upon the binding of a growth factor of the GDNF/neurturin family with a member of a family of closely related co-receptors named GRF.alpha.. A characteristic feature of the GFRco-receptor family, is that its members have no transmembrane attached to the cell surface via a glycosyl-phosphatidylinositol (GPI) linkage (Durbec et al., Nature 381:789-793, 1996; Jing et al., Cell 85:1113-1124, 1996; Treanor et al., Nature 382:80-83, 1996; Trupp et al., Nature 381:785-789, 1996; Baloh et al., 1997, supra). The members of the GRF.alpha. family include GFR.alpha.1 (previously known as GDNFR.alpha., TrnR.sub.1 and RetL1), GFR.alpha.2 (previously TrnR2, NTNR.alpha. and RetL2), GFR.alpha.3 (previously TrnR3) (GRF.alpha. Nomenclature Committee, Neuron 19(3):485, 1997) and possibly GFR.alpha.4, a receptor currently only identified in the chicken (cGFR.alpha.4) (Enokido et al., Current Biology 8, 1019-1022, 1998).
Results from in vitro experiments by multiple groups together indicate that GRF.alpha. 1/RET is the preferred receptor for GDNF, and GFR.alpha.2/RET is the preferred receptor for NTN, however cross-talk between the different receptors is possible (Baloh et al., 1997, supra; Jing et al., 1996, supra; Jing et al., J Biol. Chem. 272, 33111-33117, 1997; Klein et al., Nature 387, 717-721,1997; Sanicola et al., Proc. Natl. Acad. Sci., USA 94, 6238-6243,1997; Suvanto et al., Hum. Molec. Genet. 6, 1267-1273,1997; Treanor et al., 1996, supra). Recent analysis of GFR.alpha.1-deficient mice indicated that GFR.alpha.1 is the only physiologically relevant GDNF receptor in kidney organogenesis and enteric nervous system development (Cacalano et al., Neuron 21, 53-62,1998; Enomoto et al., Neuron 21, 317-324,1998). However, GDNF-deficient mice have greater losses in peripheral ganglia than GRF.alpha.1 -deficient mice, suggesting that GDNF can utilize additional other receptors to support survival of peripheral neurons, likely GFR.alpha.2/Ret (Cacalano et al., supra; Enomoto et al., supra). Persephin cannot signal through either the GFR.alpha.1/RET or GFR.alpha.2/RET receptor complexes (Milbrandt et al., supra), but a recent report indicated that persephin binds to cGFR.alpha.4 and likely also signals through RET (Enokido et al., supra).
GFR.alpha.3 was first identified as an expressed sequence tag (EST) that is omologous to GFR.alpha.1 and GFR.alpha.2 (Baloh et al., Proc. Natl. Acad. Sci USA 95:5801-5806, 1998, supra; Jing et al., 1997, supra; Naveilhan et al., Proc Natl Acad Sci USA 5, 1295-300, 1998; Widenfalk et al., Eur. J. Neurosci. 10, 1508-1517, 1998; Worby et al., J Biol Chem 273, 3502-3508, 1998). However, analysis in transformed cells indicated that GFR.alpha.3 could not form a functional receptor with RET for any of the known GDNF family ligands, GDNF, neurturin or persephin (Baloh et al., 1998, supra;). GFR.alpha.3 expression is much more restricted than GFR.alpha.1 or GFR.alpha.2, with high level expression observed only in developing peripheral nerve and ganglia (Baloh et al., 1998, supra; Naveilhan et al., supra; Widenfalk et al., supra; Worby et al., 1998). Furthermore, one report demonstrated that in sensory neurons of the trigeminal ganglion, GFR.alpha.3 is expressed in a population of neurons distinct from GFR.alpha.1 and GFR.alpha.2, and likely overlaps with RET (Naveilhan et al., supra). Although the structural similarity, expression, and functional data together suggested that GFR.alpha.3 can interact with RET, the work reported herein was the first to demonstrate such an interaction and the first to identify a ligand for GFR.alpha.3.
It is now generally believed that neurotrophic factors regulate many aspects of neuronal function, including survival and development in fetal life, and structural integrity and plasticity in adulthood. Since both acute nervous system injuries as well as chronic neurodegenerative diseases are characterized by structural damage and, possibly, by disease-induced apoptosis, it is likely that neurotrophic factors play some role in these afflictions. Indeed, a considerable body of evidence suggests that neurotrophic factors may be valuable therapeutic agents for treatment of these neurodegenerative conditions, which are perhaps the most socially and economically destructive diseases now afflicting our society. Nevertheless, because different neurotrophic factors can potentially act preferentially through different receptors and on different neuronal or non-neuronal cell types, there remains a continuing need for the identification of new members of neurotrophic factor families for use in the diagnosis and treatment of a variety of acute and chronic diseases of the nervous system.